1. Field of the Invention
The present invention relates to a method for achieving a clean active copper surface with good solderability (wettability) by removing dirt and a copper oxide from a copper surface before electronic components are soldered to the exposed portion (land) of a copper circuit of a printed circuit board.
2. Description of the Related Art
The following methods have been employed to maintain the solderability of a land of a printed circuit board, i.e., to protect the land from oxidation or dirt.
(1) Hot Air Leveling
In this method, a land is coated with solder (tin-lead alloy). Specifically, a printed circuit board is immersed in a solder bath and then raised from the solder bath. Unwanted solder is removed by blowing hot air on the printed circuit board, so that a thin coating of solder is formed on the land.
(2) Rosin Preflux
In this method, a land is coated with a rosin-based resin. Specifically, the rosin-based resin dissolved in a solvent is applied to a printed circuit board by a roll coater and then dried, so that a coating of rosin-based resin is formed on the land (JP 5(1993)-186713 A).
(3) Benzimidazole Preflux
In this method, a land is coated with an imidazole-based compound such as benzimidazole. Specifically, a printed circuit board is immersed in an aqueous solution of benzimidazole compound and then dried, so that a coating of benzimidazole compound is formed on the land (Japanese Patent No. 2923596).
(4) Nickel/Gold Plating
In this method, a land is coated with two layers: a nickel-plated layer and a gold-plated layer. Specifically, the land is plated successively with nickel and gold by electroplating or electroless plating.
However, each of the above conventional methods has problems. For (1), it is difficult to achieve a uniform thickness of the solder coating. For (2), the use of an organic solvent involves dangerous and harmful effects. For (3), when reflow soldering is performed at high temperatures, the coating of benzimidazole compound is deformed due to its poor heat resistance, so that solderability is reduced. For (4), nickel corrosion called “black pad” is likely to occur, the number of processes is increased, and the material cost is high.
These problems of the conventional methods are attributed to the protective coating formed on the land.